Freeze dryers of the aforementioned type are known for the drying of coffee and other comestible products and have become highly sophisticated devices designed to freeze dry, more or less continuously or semicontinuously, large quantities of comestible products in a relatively short period.
Freeze drying is based upon the principle that, under vacuum, moisture can be driven from a comestible product by heating the same to a relatively low temperature, below that at which the aesthetic and edible properties of the product are destroyed. For the most part, a conventional freeze dryer comprises a sublimation chamber having a surface upon which the comestible product is heated and an ice condenser upon which the moisture driven from the product condenses in the form of ice, i.e. sublimes from the product into a vapor of the moisture and then condenses directly as ice upon the ice condenser.
The ice condenser is traversed by a coolant or refrigerant and thus must abstract heat from the condensed material equivalent to the sublimation energy supplied to the product on the heating surface.
In the usual freeze dryer of the aforedescribed type the material to be freeze dried is disposed in shells and brought into contact with a heating plate traversed by steam, the moisture being transformed into ice on the ice condenser. The requisite low pressure is produced by a vacuum pump which evacuates the entire sublimation chamber.
Apart from high capital cost for the conventional apparatus, the system is characterized by the relatively high costs for the energy necessary to generate the cold required to freeze the moisture on the ice condenser. The cold costs are especially high when the material to be freeze dried has a comparatively low eutectic temperature (i.e. temperature at which the thawing of the material begins). For example, the juices of citrus fruits have a eutectic temperature of about minus 40.degree. C. With conventional freeze dryers the heating plate temperature must initially, in the case of coffee, be about 100.degree. C and can then drop to levels of about 50.degree. C. The temperature in the ice condenser must, for the freeze drying of coffee, be about minus 40.degree. C. As a consequence the energy required to abstract heat from the cold condenser or supply "cold" thereto is relatively high.